Teachers have faced a variety of challenges since the onset of COVID-19 in the spring of 2020, including increased workloads, a range of modalities, and COVID-19 protocols. The challenges associated with teaching during the COVID-19 pandemic have led to increased burnout and stress, as well as diminished well-being and mental health in teachers. This study seeks to explore teacher burnout and subjective mental health from the start of the pandemic to the end of the 2020-21 school year. To better understand teachers' experiences during this time, we conducted a nationwide survey (n = 302) with teachers between May and June 2021, as well as four focus groups and four one-on-one interviews in the fall of 2021. Results suggest that teachers' subjective mental health, morale, and work-life balance have significantly declined since the beginning of the pandemic. Teachers reported feeling anxious and burned out from teaching due to greater workloads and decreased support from administrators and parents. We conclude by discussing the implications of teacher subjective mental health and burnout and offer suggestions for better supporting teachers, practitioners, and their mental well-being.